SECTION III.

The Conditions which affect the Formation and Development of the Latent Image in the Collodion process.

It will be necessary to preface the observations contained in this Section by defining two terms which are frequently confounded with each other, but are in reality of distinct meaning. These terms are "Sensitiveness" and "Intensity."

By Sensitiveness is meant a facility of receiving impression from very feeble rays of light, or of receiving it quickly from brighter rays.

Intensity, on the other hand, relates to the appearance of the finished Photograph, independently of the time taken to produce it,--to the degree of opacity of the image, and the extent to which it obstructs transmitted light.

It will be seen as we proceed that the conditions necessary to obtain extreme sensitiveness of the Iodide film are different from, and often opposed to, those which give the maximum intensity of image.

CAUSES WHICH INFLUENCE THE SENSITIVENESS OF IODIDE OF SILVER ON COLLODION.

Some of the most important are as follows:--

a. The presence of free Nitrate of Silver.--When the sensitive film is removed from the Nitrate Bath, the Iodide of Silver is left in contact with excess of Nitrate of Silver. The presence of this compound is not essential to the action of the light, since, if it be removed by washing in distilled water, the image may still be impressed. In such a case however the effect is produced slowly, and a longer exposure in the Camera is required.

The sensitiveness of the Iodide film does not increase uniformly with the amount of the excess of Nitrate of Silver, as measured by the strength of the Bath. It is found that no advantage in this respect can be gained by using a proportion of Nitrate of Silver greater than 30 or 35 grains to the ounce of water, although solutions of three times this strength have been sometimes employed.

It has been asserted that a chemically pure Iodide of Silver, which is unaffected in colour by the direct action of light, is also incapable of receiving the invisible image in the Camera; and that the sensitiveness of a washed Collodion film is due to a minute quantity of Nitrate of Silver still remaining. Iodide of Silver in the state in which it is thrown down on diluting with water a strong solution of the salt known as the double Iodide of Potassium and Silver,--and which must, from the mode of its preparation, be free from Nitrate of Silver,--is quite insensitive; but this form of Iodide differs from the other in colour, and not only so, but is likely to contain an excess of Iodide of Potassium. The application of a solution of Nitrate of Silver to this compound at once renders it sensitive to light.

b. Free acids in the Nitrate Bath.--Strong oxidizing agents, such as Nitric Acid, greatly diminish the sensibility of the film, and hence the importance of removing the free acid often met with in commercial samples of the Nitrate of Silver. The effect of even a single drop of strong Nitric Acid in an eight-ounce Nitrate Bath will be appreciable; and when the proportion is increased to one drop per ounce, it will be difficult to obtain a rapid impression.

Acetic Acid has far less effect upon the sensitiveness than Nitric Acid, and being found useful during the development of the image is commonly employed; but when great rapidity is desired, it should be added cautiously, and in a proportion very much less than that in the solution known as the Aceto-Nitrate of Silver, which contains about one drop of the glacial acid to each grain of Nitrate of Silver.

c. Addition of certain organic matters.--It has long been remarked that the use of bodies like Albumen, Gelatine, Caseine, etc., which combine with Oxides of Silver, retard the action of light upon Iodide of Silver; and the recent observations of the Author enable him to confirm this statement. It is probable that one cause, amongst others, of the great sensibility of the Collodion film is due to the fact that Pyroxyline is a substance peculiarly indifferent to the Salts of Silver, exhibiting no tendency to reduce them to the metallic state; and it is proved by experiment that the addition of Grape Sugar, or of the resinous body, Glycyrrhizine, which resembles Albumen in causing a white precipitate in strong solution of Nitrate of Silver, renders necessary a longer exposure in the Camera. Alkaline Citrates have a still more marked effect, as also have Tartrates, Oxalates, etc.

d. Impurities in the soluble Iodides.--Commercial Iodide of Potassium often contains Iodate of Potash, which is found to have a retarding effect upon the action of light; also Carbonate of Potash, which, in Collodion, produces Iodoform, and in the paper processes, where "Aceto-Nitrate" is used for sensitizing, forms Acetate of Silver. Iodoform has a marked influence in diminishing the sensitiveness of Iodide of Silver; Acetate of Silver may perhaps increase it a little by securing the absence of free Nitric Acid (p. 117). Iodide of Potassium prepared by the process in which Sulphuretted Hydrogen and Alcohol are used, and having a smell of Garlic, contains probably Xanthate of Potash, and is nearly useless for Photography.

See the Vocabulary, Part III., Art. Iodoform.

Commercial Iodide of Cadmium is a purer salt than the Iodide of Potassium, and may be advantageously substituted for it; but it possesses the property of coagulating Albumen, and hence cannot be employed in conjunction with that substance.

e. Presence of free Iodine.--Both in the waxed paper and the Collodion processes, the solutions often contain a small quantity of free Iodine. This Iodine, in contact with the Nitrate of Silver of the Bath, produces a mixed Iodide and Iodate of Silver, and liberates Nitric Acid. It thus retards the sensitiveness of the film in proportion to the quantity of Iodine present. Collodion of a full yellow colour is perceptibly less sensitive than the same rendered colourless; and when enough Iodine has been liberated to give a red or brown tint, double the original exposure will probably be required.

If brown Collodion be much used, the Nitrate Bath may by degrees become sufficiently contaminated with free Nitric Acid to interfere with the sensitiveness of the film; but if colourless or lemon-yellow tinted Collodion be employed, this evil need not be anticipated.

Certain substances may be added to coloured Collodion which possess the property of counteracting the retarding influence of the free Iodine, such, for instance, as the Oils of Cloves, Cinnamon, etc.; they probably act in virtue of their affinity for Oxygen, by preventing the formation of Iodate of Silver. In colourless Collodion they produce little or no effect, neither do they remove the insensitiveness of the film when dependent upon a too acid condition of the Nitrate Bath.

f. Addition of Bromide or Chloride to Collodion.--In the Daguerreotype a very exalted state of sensibility is obtained by exposing the silvered plate first to the vapour of Iodine, and afterwards to that of Bromine or Chlorine; but this rule does not apply to the Collodion process, which differs essentially in principle. Soluble Bromides added to Collodion lessen its sensibility to an appreciable extent, as also do Chlorides. This rule however may perhaps be liable to an exception when artificial light is used, which contains a greater proportion of the rays of small refrangibility, known to act more powerfully upon the Bromide than upon the Iodide of Silver (p. 66).

g. Density of the sensitive film.--When the proportion of soluble Iodide in the iodizing solution is too great, the film is very dense, and the Iodide of Silver is apt to burst out upon the surface, and fall away in loose flakes into the Bath. This condition, which is highly unfavourable to sensitiveness, is very common in Collodion, and constitutes what is termed "over-iodizing." The Iodide, in fact, is formed in such a case too much upon the surface, and consequently, when the fixing agent is applied, the image not being retained by the film, is washed off and lost.

On the other hand, the sensibility of the film is not lessened by reducing the amount of Iodide in Collodion to a minimum, if all the solutions are neutral; but the pale blue films formed by a dilute Collodion, and which almost rival the Daguerreotype itself in delicacy, are nearly useless in practice; for if free Iodine or other bodies of a retarding nature are present in any quantity, either in the Collodion or in the Bath, they almost destroy the action of a weak light, producing a far more injurious effect than if the film were more yellow and opaque.

h. Impurities in Ether and Alcohol.--Pure Ether should be neutral to test-paper, but the commercial samples of this article have usually either an acid or an alkaline reaction. The frequent occurrence of a peculiar oxidizing principle in Ether has also been pointed out (p. 85). Each of these three conditions is injurious to sensitiveness; the first and last by liberating Iodine when alkaline Iodides are used; and the second, by producing Iodoform under the same circumstances. In this case the Collodion remains colourless, but gives inferior results.

The Author has also observed that Ether which has been redistilled from the residues of Collodion may contain a volatile principle (probably a compound Ether?) which produces a retarding effect upon the action of light.

Commercial Spirit of Wine is not always uniform in composition, as sufficiently evidenced by the test of smell. It may contain "fusel oil" or other volatile substances, which become milky on dilution with water, and are believed to injure the quality of the spirit for Photographic use.

i. Relative proportions of Ether and Alcohol in Collodion.--It was shown at p. 84 that the addition of Alcohol to Collodion lessens the contractility of the film, and renders it soft and gelatinous. This condition is favourable to the formation of the invisible image in the Camera, the play of affinities being promoted by the loose manner in which the particles of Iodide are held together. It is therefore usual to add to Collodion as much Alcohol as it will bear without becoming glutinous, or leaving the glass; the exact quantity required varying with the strength of the spirit or its freedom from dilution with water.

k. Decomposition in the Collodion.--Collodion iodized with the metallic Iodides generally, excepting the Iodide of Cadmium, becomes brown and loses sensitiveness in the course of a few days or weeks. If the free Iodine, the cause of the brown colour, be removed, the greater part, but not the whole, of the sensitiveness is regained. The experiments of the Author, and of others, have proved that a solution of Pyroxyline in contact with an unstable iodide, slowly undergoes decomposition, the result of which is that Iodine is set free, and an equivalent quantity of the base remains in union with certain organic elements of the Collodion.

Decomposition also gradually ensues when iodized Collodion is placed in contact with reducing agents, such as Proto-iodide of Iron, Gallic Acid, Grape Sugar, Glycyrrhizine, etc., so that these combinations do not retain a constant sensibility for any length of time. Even plain Collodion uniodized cannot be preserved many months without a small but perceptible amount of change.

l. Decomposition in the Nitrate Bath.--A Collodion Nitrate Bath which has been much used, often gives a less sensitive film than when newly made. It is known also that many organic substances which reduce Nitrate of Silver, if added to the Bath, produce a state which is favourable to sensitiveness whilst the decomposition is taking place, but is eventually unfavourable; hence the solution will be injured by adding either Gallic or Pyrogallic Acid, and by organic matters generally if exposed to light.

Recapitulation.--The conditions most favourable to extreme sensitiveness of the Iodide of Silver on Collodion may be condensed as follows:--perfect neutrality of the solutions employed; a soft, gelatinous state of the film; absence of Chlorides and other salts which precipitate Nitrate of Silver; an undecomposed Collodion, containing no organic matter of that kind which is precipitated by basic Acetate of Lead, and combines with oxides of Silver.

THE CONDITIONS WHICH AFFECT THE DEVELOPMENT OF THE LATENT IMAGE.

The general theory of the development of a latent image by means of a reducing agent, having been simply explained in the third Chapter, may now be more fully examined in its application to the Iodide of Silver on Collodion.

a. The presence of free Nitrate of Silver essential to the development.--This subject has already been mentioned (p. 36). A sensitive Collodion plate, carefully washed in distilled water, is still capable of receiving the radiant impression in the Camera, but it does not admit of development until it has been redipped in the Bath, or treated with a reducing agent to which Nitrate of Silver has been added: and if the proportion of free Nitrate of Silver on a Collodion film be too small, the image will be feeble or altogether imperfect in parts, with patches of green or blue, due to deficient reduction.

b. Comparative strength of deducing Agents.--No increase of power in a developer will suffice to bring out a perfect image on an under-exposed plate, or upon a film containing too little Nitrate of Silver. But there is considerable difference in the length of time which the various developers require to act. Gallic Acid is the most feeble, and Pyrogallic Acid the strongest, producing at least four times more effect than an equal weight of the crystallized Protosulphate of Iron, and twenty times more than the Protonitrate of Iron.

c. The effect of free Acid upon the development.--Acids tend to retard the reduction of the image as well as to diminish the sensibility of the film to light. Nitric Acid especially does so, from its powerful oxidizing and solvent properties. The effect of Nitric Acid is particularly seen when the film of Iodide of Silver is very blue and transparent, and the quantity of Nitrate of Silver retained upon its surface small. Under such circumstances the proper development of the image may be suspended, and spangles of metallic Silver separate. This indicates that the quantity of the acid should be diminished, or the strength of the Nitrate Bath and of the reducing agent be increased, as a counterpoise to the retarding action of acid upon the development.

Acetic Acid also moderates the rapidity of development, but it has not that tendency altogether to suspend it possessed by Nitric Acid. It is therefore usefully employed, to enable the operator to cover the plate evenly with liquid before the development commences, and to preserve the white parts of the impression from any accidental deposit of metallic Silver due to irregular action of the reducing agent.

On comparing the retarding effects of free acid upon the light's action, and upon the development, we see that the former is the most marked,--that a small quantity of Nitric Acid produces a more decided influence upon the impression of the image in the Camera than upon the bringing out of that image by means of a developer.

d. Accelerating effect of certain organic matters.--Organic bodies, like Albumen, Gelatine, Glycyrrhizine, etc., which combine chemically with oxides of Silver, and were shown in the last Section to lessen the sensitiveness of the Iodide film,--facilitate the development of the image, producing often a dense deposit of a brown or black colour by transmitted light.

In the same way, viz. by a retention of organic matter, may partly be explained the fact, that the image developed by Pyrogallic Acid, although proved by the application of tests to contain no more than an equal quantity of Silver, possesses greater opacity by transmitted light, than that resulting from the use of protosalts of Iron: and in the case of the Collodion itself the same rule applied--if it be pure, it is liable to give a less vigorous impression than when by long keeping a partial decomposition has taken place, and products have been formed which combine with reduced Oxide of Silver more easily than the unaltered Pyroxyline.

e. Molecular conditions affecting Intensity.--The physical structure of the Collodion film is thought to exert an influence upon the mode in which the reduced Silver is thrown down during the development. A short and almost powdery state, such as Collodion iodized with the alkaline iodides acquires by keeping, is considered favourable, and a glutinous, coherent structure unfavourable, to density. This is certainly the case when the film is allowed to dry before development, as in the process with desiccated Collodion and, to some extent, in the Oxymel preservative process.

The mode of conducting the development also affects the density; a rapid action tending to produce an image of which the particles are finely divided and offer a considerable resistance to the passage of light, whilst a slow and prolonged development often leaves a metallic and almost crystalline deposit, comparatively translucent and feeble.

The writer has observed, that with certain samples of Collodion the image is much enfeebled by keeping the plate for a considerable time,--a quarter of an hour or longer,--after sensitizing, but before development. This effect is not the result of the Nitrate of Silver having partially drained away, since a second dip in the Nitrate Bath immediately before applying the Pyrogallic Acid, does not remedy it. An alteration of molecular structure may therefore be the correct explanation, and if so, a contractile Collodion would suffer more than one possessing less coherency.

The actinic power of the light at the time of taking the picture, influences the appearance of the developed image; the most vigorous impressions being produced by a strong light acting for a short time. On a dull dark day, or in copying badly lighted interiors, the photograph will often lack bloom and richness, and be blue and inky by transmitted light.

f. Development of images upon Bromide and Chloride of Silver.--Of the three principal Salts of Silver, the Iodide is the most sensitive to light, but the Bromide and Chloride, under some conditions, are more easily developed and give a darker image. In the Collodion process the difference is principally seen when organic bodies, like Grape Sugar, Glycyrrhizine, etc., are introduced in order to increase the intensity; a far more decided effect being produced by adding both Glycyrrhizine and a portion of Bromide or Chloride, than by using the Glycyrrhizine alone.

See the Author's Paper on the chemical composition of the photographic image, in the eighth Chapter.

g. The intensity of the image affected by the length of exposure.--This point has been briefly alluded to in the third Chapter. If the exposure in the Camera be prolonged beyond the proper time the development takes place rapidly but without any intensity, the picture being pale and translucent. The effects produced by over-action of the light are particularly seen when the Nitrate Bath contains Nitrite of Silver, or Acetate of Silver; the image being frequently in such a case dark by reflected light, and red by transmitted light,--more nearly resembling in fact a photographic print, developed on paper prepared with Chloride of Silver. When Collodion plates are coated with honey without previously removing the free Nitrate of Silver, a slow reducing action is set up, which may give rise to the characteristic appearance above referred to, after development. Other organic substances, such as biliary matter, etc., will act in the same way.

h. Certain conditions of the Bath which affect development.--Attention may be called to a peculiar state of the Nitrate Bath, in which the Collodion image developes unusually slowly, and has a dull grey metallic appearance, with an absence of intensity in the parts most acted on by the light. This condition, which occurs only when using a newly mixed solution, is thought by the Author to depend upon the presence of an Oxide of Nitrogen retained by the Nitrate of Silver. It is removed partially by neutralizing the Bath with an alkali, more perfectly so by adding an excess of alkali followed by Acetic Acid; but most completely by carefully fusing the Nitrate of Silver before dissolving it.

Commercial Nitrate of Silver has sometimes a fragrant smell, similar to that produced by pouring strong Nitric Acid upon Alcohol. When such is the case, it contains organic matter, and produces a Bath which yields red and misty pictures.

Nitrate of Silver which has been sufficiently strongly fused to decompose the Salt, and produce a portion of the basic Nitrite of Silver exhibits great peculiarity of development, the image coming out instantaneously and with great force. This condition is exactly the reverse of that produced by the presence of acids, in which the development is slow and gradual.

In summing up the different conditions of the Nitrate Bath which affect the development of the image, as many as four might be mentioned, each of which gives a more rapid reduction than the one which precedes it. These are--the acid Nitrate Bath, the neutral Bath, the Bath of strongly fused Nitrate of Silver, and the Bath containing Ammoniacal Nitrate of Silver, which is quite unmanageable, and produces an instantaneous and universal blackening of the film on the application of the developer.

Greater intensity of image is commonly obtained in a Nitrate Bath which has been a long time in use, than in a newly mixed solution: this may be due to minute quantities of organic matter dissolved out of the Collodion film, which, having an affinity for Oxygen, partially reduce the Nitrate of Silver; and also to the accumulation of Alcohol and Ether in an old Bath producing a short and friable structure of the film.

i. Effect of Temperature on Development.--Reduction of the oxides of noble metals proceeds more rapidly in proportion as the temperature rises. In cold weather it will be found that the development of the image is slower than usual, and that greater strength of the reducing agent and more free Nitrate of Silver is required to produce the effect.

On the other hand, if the heat of the atmosphere be excessive, the tendency to rapid reduction will be greatly increased, the solutions decomposing each other almost immediately on mixing. In this case the remedy will be to use Acetic Acid freely both in the Bath and in the developer, at the same time lessening the quantity of Pyrogallic Acid, and omitting the Nitrate of Silver which is sometimes added towards the end of the development.

Also in the case of films which are to be kept for a long time in a sensitive condition by means of honey, etc., the modifying influence of temperature must be observed, and the quantity of free Nitrate of Silver left upon the film be reduced to a minimum if the thermometer stands higher than usual.